Observational Constraints on The Evolution of Dust in ...

Evolution of Dust in Protoplanetary Disks 137
that grains coagulate faster in the inner disk where dynamical timescales are shorter.
However as discussed by Oliveira et al. (2010) and in § 6.5.1, the mean dust size at
the disk surface is not regulated by grain growth alone, but also by fragmentation
and vertical mix. This means that faster coagulation at smaller radii cannot be
uniquely responsible for bigger grains in the inner disk. Future modeling should try
to understand this difference in mean grain sizes observed.
Figure 6.3 – Distribution of mass-averaged mean grain sizes for the warm (〈a warm〉, left panel) and
cold (〈a cold 〉, right panel) components. Due to the low number statistics, the objects in Upper Sco
and η Cha have been merged together as an older cluster. (A color version of this figure is available
in the online journal)
Furthermore, Serpens and Taurus occupy an indistinguishable locus in Figure 6.2,
explicitly seen in Figure 6.3. A two sample Kolmogorov-Smirnov test (KS-test) was
performed and the results show that the null hypothesis that the two distributions
come from the same parent population cannot be rejected to any significance (14%).
The older regions, although lacking statistical significance, show a distribution of
mass-average grain sizes in the same range probed by the young star-forming regions
(Figure 6.3). This supports the evidence that the size distribution of the dust in the
surface layers of disks is statistically the same independent of the population studied
(Oliveira et al. 2010).
The results here confirm those from Olofsson et al. (2010) that the mean differential
grain size distributions slope for the three grain sizes considered are shallower than
the reference MRN differential size distribution (α = -3.5). The mean grain size
distributions slopes (α) for each region can be found in Table 6.3.